From de2e90b3cb3598c2a992992ce197a04a0c8de916 Mon Sep 17 00:00:00 2001
From: Marcus Ottosson <konstruktion@gmail.com>
Date: Tue, 11 Aug 2015 13:27:03 +0100
Subject: [PATCH] Fixing #219

---
 pyblish/plugin.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/pyblish/plugin.py b/pyblish/plugin.py
index 001d7713..cd0400ac 100644
--- a/pyblish/plugin.py
+++ b/pyblish/plugin.py
@@ -573,12 +573,15 @@ def __init__(self, *args, **kwargs):
         self._children = dict()
 
     def add(self, other):
-        super(Context, self).add(other)
+        if other in self:
+            raise ValueError("\"%s\" already in Context" % other)
+
         self._children[other.id] = other
+        super(Context, self).add(other)
 
     def remove(self, other):
-        super(Context, self).remove(other)
         self._children.pop(other.id)
+        super(Context, self).remove(other)
 
     def create_instance(self, name, **kwargs):
         """Convenience method of the following.